A defective variant of simian virus 40 containing covalently linked DNA sequences derived from the host (African Green monkey) genome is being used to study the origin of such substituted defectives as well as the structure of monkey DNA. The nucleotide sequence of a segment of the defective genome containing monkey sequences (185 base pairs) has been determined. The segment from the defective hybridizes, in situ to both centromeric and non-centromeric regions of African Green monkey chromosomes. The sequence in the defective is derived from highly repeated monkey DNA and occurs almost 2 million times in the monkey genome. Cleavage of total monkey DNA with restriction endonuclease HindIII permits isolation of a 172 base pair long fragment representing about 7 percent of the total DNA. The nucleotide sequence of this fragment was also determined. The two fragments, one from the defective and one from monkey DNA have 156 base pairs in common. The fragment from the defective joins, on both ends, sequences not found in the segment isolated from the monkey DNA. BIBLIOGRAPHIC REFERENCES: Singer, M.F.: Summary of the National Institutes of Health (USA) Guidelines for Recombinant DNA Research. Gene 1: 123-139, 1977. Rao, G.R.K. and Singer, M.F.: Studies on a defective variant of simian virus 40 that is substituted with DNA sequences derived from monkey DNA I. Origin, properties, and purification. J. Biol. Chem. In press, 1977.